An electrophotographic image forming apparatus has found wide acceptance in not only a copying machine but also a printer, an output device of a computer which has been increasingly demanded in recent years. In the electrophotographic image forming apparatus, a photosensitive layer of an electrophotographic photoreceptor installed in the apparatus is uniformly charged with a charging unit, exposed to, for example, a laser beam corresponding to an image information, and a fine-grain developer called a toner is supplied to an electrostatic latent image formed by the exposure from a developing unit to form a toner image. The toner image is subjected to a transfer process before fixed to paper (medium) by a heat fuser.
The toner image formed by a developer-component toner attaching on the surface of an electrophotographic photoreceptor is transferred by transfer means to a transfer material such as recording paper. However, the toner on the surface of the electrophotographic photoreceptor is not entirely moved to the recording paper through transfer as such but is partially left on the surface of the electrophotographic photoreceptor. Such toner particles remained on the surface of the electrophotographic photoreceptor adversely affect the quality of the resulting image, and thus are eliminated by a cleaning device.
In recent years, such an electrophotographic image forming apparatus has become popular for use as not only monochrome but also as color output means, and the demand for higher-quality image formation is ever more increasing. As means for increasing the image quality, various proposals have been so far made specifically for image formation processes. The typical means therefor is reducing the particle size of toner and carrier, both of which are a developer component for use in a developing process of forming toner images by developing electrostatic latent images.
Reducing the particle size of the developer-component toner and carrier as such can increase the image quality with the better tone of images, reproducibility of thin lines, and density uniformity of solid-filled areas using a finely-manufactured magnetic brush in the developing means. What is more, with the image forming apparatus which is becoming smaller in size and faster in image formation processing speed, the level of stresses applied to a developer is reduced as the carrier is reduced in weight. As such, also in terms of durability, reducing the particle size of the carrier is considered preferable.
The problem here is that reducing the particle size of the toner causes the transfer efficiency to be lowered. This is because the toner particles are increased in attachment strength with respect to the electrophotographic photoreceptor due to image force, Van der Waals force, or others. As a result, image transfer to the transfer material becomes difficult so that the transfer efficiency is resultantly reduced. In consideration thereof, the size-reduced toner particles are shaped much rounder, and the resulting toner particles are reduced in area for contact with the surface of the electrophotographic photoreceptor so that the attachment strength is controlled. In this manner, the transfer efficiency and the image quality are both increased. Because shaping the toner particles rounder favorably increases the transfer efficiency, the toner consumption is reduced in amount per piece for copying, and the toner particles to be left in the apparatus after image transfer are reduced in amount. Accordingly, this enables beneficial image formation in view of lower cost and energy saving.
Moreover, a tendency is observed that the electrical charge density of the toner particles is intensely higher at the protrusion portion of the particles. It means that, with the higher average roundness as a result of rounding the toner particles, such nonuniformity is not observed any more to the electrical charge density of the toner particles, whereby the electrical charge characteristics are stabilized. As a result, the difference of the electrical charge characteristics is reduced between the toner particles, and this makes the amount distribution of electrical charge in the toner in its entirety, thereby achieving the higher image quality. What is more, in the rounded toner particles, the percentage occupied by the protrusion portion is less. The rubbing friction between the toner particles and the surface of the electrophotographic photoreceptor becomes thus low, and the surface of the electrophotographic photoreceptor is controlled not to suffer from film abrasion.
The issue here is that reducing the particle size of the toner and the carrier problematically causes a problem of so-called poor cleaning in a cleaning process, which is executed to eliminate any toner particles remaining on the surface of the electrophotographic photoreceptor after toner image transfer to the transfer material. Here, the poor cleaning is a phenomenon affecting the image formation process of the following cycles. This is caused by the elimination failure in the cleaning process with respect to the toner particles, which are partly left on the surface of the electrophotographic photoreceptor as a result of the transfer failure in the transfer process from the electrophotographic photoreceptor to the transfer material. To be more specific, it is the phenomenon of toner leak lines in the rotation direction of the electrophotographic photoreceptor or white fogging on the image.
As the toner particles are reduced in size, the specific surface being the surface area of the toner per unit weight is increased. This increases the effects of the intermolecular forces acting on with the electrophotographic photoreceptor per toner particle, thereby decreasing the level of cleaning performance.
The toner particles originally have the large attachment energy with respect to the surface of the electrophotographic photoreceptor. Therefore, as the average roundness is increased due to the toner particles shaped rounder, the toner particles are not scraped by a cleaning blade when the surface of the electrophotographic photoreceptor is subjected to cleaning using the cleaning blade. It means that the toner particles pass through between the edge of the cleaning blade and the surface of the electrophotographic photoreceptor with ease, resultantly the cleaning performance is problematically decreased to a further degree.
As a result of size reduction of the toner particles, such a phenomenon of the decreased cleaning performance with respect to the electrophotographic photoreceptor may be resulted from mutual attachment therebetween, associated with the size of the toner particles and the surface condition of the electrophotographic photoreceptor. In view thereof, to increase the cleaning performance of the electrophotographic photo receptor in a case of using the size-reduced toner particles, there needs to control the cleaning performance with a consideration to the surface condition of the electrophotographic photoreceptor itself.
Such a phenomenon of poor cleaning may be resulted from mutual attachment, associated with the condition of the toner particles and the surface condition of the electrophotographic photoreceptor. In view thereof, to increase the cleaning performance of the electrophotographic photoreceptor, there needs to control the cleaning performance with a consideration to the surface condition of the electrophotographic photoreceptor itself.
The most important function of the cleaning device is not to leave any toner particle on the electrophotographic photoreceptor. In addition thereto, the cleaning device is also required not to damage the electrophotographic photoreceptor, not to bring in a single foreign substance to the toner particles in the collected toner, and not to cause the cleaning features to change over a long period of time. Such a cleaning device often adopts a method of using a fast-rotating fur brush or a WEP paper sheet, for example, and generally a blade cleaning method in which a cleaning blade abuts on the electrophotographic photoreceptor to make it slide in contact therewith.
As to the process of fixing a toner image after it is transferred to a paper sheet or others during image formation, various types of methods and apparatuses have been proposed. Currently, the most general method for toner image fixation is of crimp-and-heat using a heat roller. With this crimp-and-heat method using a heat roller, the side of a toner image on the to-be-fixed sheet is made contact with the surface of the heat roller under pressure, and the roller rolls thereover for image fixation. The surface of the heat roller is made of a material that is releasable from the toner. With this crimp-and-heat method, the surface of the heat roller is made contact with the toner image on the to-be-fixed sheet under pressure. Accordingly, the heat efficiency is quite good when the toner image is fused onto the to-be-fixed sheet, enabling swift image fixing. This is considered quite effective with high-speed electrophotographic copying machines.
The issue here is that, with such a crimp-and-heat method, there needs to fix the toner image onto the to-be-fixed sheet in a short time while the heat roller is rolling thereover. Therefore, the heat roller has to be heated high in temperature. This means that the consumption energy at the time of operation of the copying machine and the printer is mostly consumed in the image fixation process.
In recent years, under the circumstances that the energy saving is in demand to decrease the loads to the global environment, reducing such a consumption energy in the image fixation process is a significant issue. In order to meet such a demand of energy saving, proposed is a low-temperature fusing toner, which can be fused at a lower temperature compared with the conventional toner. By using such a low-temperature fusing toner, it becomes possible to reduce the consumption energy in the image fixation process. The problem with the low-temperature fusing toner is that it is easily stuck to the surface of the electrophotographic photoreceptor as it is soft and has a lower-melting point compared with the conventional toner, thereby easily causing filming disadvantageously.
For solution of such problems, there is a method of eliminating the remaining toner particles and filming-occurred toner on the electrophotographic photoreceptor by increasing the abutment pressure (the load per unit length, and hereinafter referred to as line voltage) of the cleaning blade to the electrophotographic photoreceptor. The problem with this method is that increasing the line voltage surely increases the cleaning performance but also causes abrasion of a photosensitive layer of the electrophotographic photoreceptor, thereby shorting the useful life of the electrophotographic photoreceptor.
Further, in an attempt to improve the toner quality, together with the above-described low-temperature image fixation, proposed is to shape the toner particles rounder for the purpose of improving the image quality and achieving the low cost. By shaping the toner particles rounder as such, the toner particles are reduced in area of abutting on the surface of the electrophotographic photoreceptor, and thereby the attachment strength thereof can be reduced. As a result, the transfer efficiency of the toner is increased, and an amount of the toner used is reduced per image for formation so that the cost for image formation is reduced. What is more, because the toner particles become uniformly charged, the reproducibility of thin lines and others of the image can be increased. The issue here is that the rounder toner particles are difficult to be scraped by the cleaning blade at the time of cleaning, thereby problematically resulting in the poor cleaning result.
The phenomenon of poor cleaning of the electrophotographic photoreceptor as a result of temperature reduction for fixation of the toner particles and rounder shape formation thereof may be resulted from mutual attachment, associated with the toner particles and the surface condition of the electrophotographic photoreceptor. In view thereof, to increase the cleaning performance of the electrophotographic photoreceptor, there needs to go through development with a consideration to the surface condition of the electrophotographic photoreceptor itself.
Cleaning of the electrophotographic photoreceptor is to eliminate any remaining toner particles with a force acting thereon from the surface of the electrophotographic photoreceptor. The force is the one exceeding the attachment strength between the surface of the electrophotographic photoreceptor and the remaining toner particles attached thereon.
Accordingly, the lower the wettability of the surface of the electrophotographic photoreceptor, the easier the cleaning. The wettability, namely, the adhesion of the surface of the electrophotographic photoreceptor can be expressed using a surface free energy (which has the same meaning as a surface tension) as an index. The surface free energy (γ) is a phenomenon which an intermolecular force, a force acting between molecules constituting a substance, causes on the outermost surface.
A toner that remains on the surface of the electrophotographic photoreceptor by adhesion or fusion without being transferred onto a transfer member is spread on the surface of the electrophotographic photoreceptor in the form of a film while steps from charging to cleaning are repeated. This phenomenon corresponds to “adhesion wettability” in the wettability.
FIG. 5 is a side view showing a state of adhesion wettability. In the adhesion wettability shown in FIG. 5, the relation between the wettability and the surface free energy (γ) is represented by Young's formula (1).γ1=γ2·cos θ+γ12   (1)
wherein    γ1: surface free energy on a surface of product 1    γ2: surface free energy on a surface of product 2    γ12: interface free energy of products 1 and 2    θ: contact angle of product 2 to product 1
In formula (1), reduction in wettability of product 2 to product 1 which means that θ is increased for less wetting is attained by increasing the interface free energy Y12 related with a wetting work of the electrophotographic photoreceptor and the foreign matters and decreasing the surface free energies γ1 and γ2.
When adhesion of foreign matters, a toner to the surface of the electrophotographic photoreceptor is considered in formula (1), product 1 corresponds to the electrophotographic photoreceptor and product 2 to a toner respectively. Accordingly, when the electrophotographic photoreceptor is actually cleaned, the wettability on the right side of formula (1), namely, the adhered condition of the toner to the electrophotographic photoreceptor can be controlled by controlling the surface free energy γ1 of the electrophotographic photoreceptor.
In the prior technique that defines a surface condition of an electrophotographic photoreceptor, a contact angle with pure water is used (refer to, for example, Japanese Unexamined Patent Publication JP-A 60-22131 (1985)). However, in regard to wetting of a solid and a liquid, the contact angle θ can be measured as shown in FIG. 5, but in case of a solid and a solid such as an electrophotographic photoreceptor and a toner, the contact angle θ cannot be measured. Accordingly, the foregoing prior technique can be applied to wettability between a surface of an electrophotographic photoreceptor and pure water, but a relation between wettability and cleanability of a solid such as a toner cannot be explained satisfactorily.
With respect to an interface free energy between a solid and a solid which is deemed necessary for evaluation of a wettability between a solid and a solid, the Forkes's theory stating a non-polar intermolecular force is considered to be further extended to a component formed by a polar or hydrogen-bonding intermolecular force (refer to Kitazaki T., Hata T., et al.; “Extension of Forkes's Formula and Evaluation of Surface Tension of Polymeric Solid”, Nippon Secchaku Kyokaishi, Nippon Secchaku Kyokai, 1972, vol. 8, No. 3, pp. 131-141). According to this extended Forkes's theory, the surface free energy of each product is found from2 to 3 components. The surface free energy in the adhesion wettability corresponding to the adhesion of the toner to the surface of the electrophotographic photoreceptor can be found from 3 components.
The surface free energy between solid products is described below. In the extended Forkes's theory, an addition rule of the surface free energy represented by formula (2) is assumed to be established.γ=γd+γp+γh   (2)
wherein    γd: dispersion component (non-polar wettability)    γp: dipolar component (polar wettability)    γh: hydrogen-bonding component (hydrogen-bonding wettability)
When the addition rule of formula (2) is applied to the Forkes's theory, the interface free energy γ12 between product 1 and product 2 which are both solids is obtained as shown in formula (3).γ12=γ1+γ2−{2√(γ1d·γ2d)+2√(γ1p·γ2p)+2√(γ1h·γ2h)}  (3)
wherein    γ1: surface free energy of product 1    γ2: surface free energy of product 2    γ1d, γ2d: dispersion components of product 1 and product 2    γ1p, γ2p: dipolar components of product 1 and product 2    γ1h, γ2h: hydrogen-bonding components of product 1 and product 2
The surface free energies (γd, γp, γh) of the components in the solid products to be measured as represented by formula (2) can be calculated by using known reagents and measuring adhesion with the reagents. Accordingly, with respect to product 1 and product 2, it is possible that the surface free energies of the components are found and the interface free energy of product 1 and product 2 can be found from the surface free energies of the components using formula (3).
The technique of increasing the cleaning performance and the durability of the electrophotographic photoreceptor is disclosed as the related art (e.g., refer to Japanese Unexamined Patent Publications JP-A 2002-131957, JP-A 2002-229234, and JP-A 2002-304022). That is, based on the concept of the interfacial free energy between the solid substances calculated as such, the surface free energy (γ) of the electrophotographic photoreceptor including a photoconductive layer of amorphous Si is defined to be 35 to 65 mN/m or 35 to 55 mN/m, and the average diameter of the toner particles is defined to be 3 to 11 μm or 4 to 10 μm.
As to the electrophotographic photoreceptor including a photoconductive layer of an organic photosensitive material, the technique of increasing the cleaning performance on the surface of the electrophotographic photoreceptor, and achieving the longer useful life thereof by defining the surface free energy to be in the range from 35 to 65 mN/m is also disclosed in the related art (refer to Japanese Unexamined Patent Publication JP-A 11-311875 (1999)).
However, the inventors of the present invention use the electrophotographic photoreceptor having the surface free energy (γ) of 35 to 65 mN/m being the range disclosed in the related arts to conduct an actual performance test by actually forming an image with respect to a recording paper. As a result of such a test study, the surface of the electrophotographic photoreceptor is observed with flaws that are possibly resulted from exposure to foreign substances such as paper powder. Also observed on the image transferred to the recording paper are black streaks resulted from poor cleaning due to those flaws.
In still the related art which is disclosed in JP-A 11-311875, an amount (Δγ) of change in surface free energy according to duration of an electrophotographic photoreceptor is defined. However, in consideration of the facts that the amount (Δγ) of change is not determined by defining initial characteristics, for example, the surface free energy, of the electrophotographic photoreceptor and the amount (Δγ) of change varies depending on conditions such as an environment in image formation and a material of a transfer member, the amount (Δγ) of change is problematic in that it might include an uncertain element and is therefore inappropriate as a designing standard in actual designing of an electrophotographic photoreceptor.
The related art about increasing the quality and the resolution of to-be-formed images includes the following techniques. The one technique is of defining the volume average diameter of magnetic toner particles to be 4 to 9 μm, providing specific inorganic particles into the very surface layer of the electrophotographic photoreceptor, and defining the surface roughness Rz to be 0.1 to 1.0 μm (refer to Japanese Unexamined Patent Publication JP-A9-152775 (1997)). The other technique is of defining the volume average diameter of toner particles to be 5 to 10 μm and the volume average diameter of carriers to be 15 to 45 μm, and defining the relationship between the surface friction coefficient of the electrophotographic photoreceptor and the kinetic friction coefficient of a magnetic brush (refer to Japanese Unexamined Patent Publication JP-A 2002-207304).
The concern here is that neither JP-A 9-152775 nor JP-A 2002-207304 discloses a technique of solving the decreasing cleaning performance resulted from particle size reduction as described above. Moreover, with the technique disclosed in JP-A 9-152775, there needs to prepare an electrophotographic photoreceptor whose very surface has specific inorganic particles scattered thereon. This raises a problem in view of productivity.
There are still other related arts, and one is proposing a technique of increasing the cleaning performance and deriving high-quality images with stability by defining the surface free energy to be 40 to 80 mN/m for the electrophotographic photoreceptor including a layer of siloxane resin serving as a surface protection layer, by defining the average diameter of the toner particle to be 4 to 12 μm, and by defining the average amount of electrical charge (refer to Japanese Unexamined Patent Publication JP-A 2001-272809). The problem with the technique disclosed in JP-A 2001-272809 is that the sensitivity and the electrification stability are not practically enough due to such a structure that the protection layer is placed on the surface of the electrophotographic photoreceptor. What is more, the production efficiency is not good.
The related art of proposing to increase the quality of images by shaping the toner particles rounder uses a magnetic toner to derive images with extremely little fogging (refer to Japanese Unexamined Patent Publication JP-A 2001-235899). The magnetic toner is the one including inorganic fine powder and conductive powder on the surfaces of magnetic toner particles including a bonding resin and a magnetic substance. By defining the average roundness of such magnetic toner particles to be 0.970 or more, every particle of the magnetic toner becomes uniformly charged. However, JP-A2001-235899is not disclosing a technique of solving the problem of causing poor cleaning, resulting from the fact that, as the average roundness of the toner particles is increased, the remaining toner particles can easily pass through between the edge of the cleaning blade and the surface of the electrophotographic photoreceptor.
There is still another technique of achieving energy saving and preventing filming by using a toner having a specific glass transition temperature (Tg) with respect to the electrophotographic photoreceptor with the surface free energy (γ) of 35 to 65 mN/m (refer to JP-A2002-131957) . In the technique disclosed in JP-A 2002-131957, however, the electrophotographic photoreceptor is limited to be of amorphous silicon. Although the amorphous-silicon photoreceptor has good hardness and can achieve the long user life, it is quite high in manufacture cost compared with an organic electrophotographic photoreceptor. Further, compared with a multi-layered organic electrophotographic photoreceptor varying in material type for selection and in characteristics, the design flexibility is narrower.